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• 2. Post-dam History of Ecosystem and Fishery: Closure of Glen Canyon Dam in <br />1963 began filling Lake Powell and created a new environment that drastically <br />altered the habitat used by fish. The warm, shallow river gave way to a <br />clear, deep, thermally stratified reservoir. Nonnative species became <br />established by intentional and unintentional introductions (Appendix A.). <br />Water released from the dam created cold water habitat which was stocked with <br />trout. Largemouth bass and crappie populations were stocked initially and <br />subsequently proliferated to provide the bulk of the sport fisheries in the <br />filling reservoir. Both species have declined in recent years due to lack of <br />brush shelter for young fish. Filling, fluctuation and aging of the reservoir <br />resulted in changing habitat that favored different species. The filling <br />reservoir had ample terrestrial vegetation that became great fish habitat when <br />inundated by each succeeding spring flood. Centrarchids that use vegetation <br />(largemouth bass, crappie, bluegill) proliferated under these conditions. But <br />when the lake filled and subsequently declined, the brush was gone and brush <br />loving fish declined. They were replaced by fish that could use the remaining <br />ubiquitous rocky structure and open water more effectively. The habitat <br />change led to the introduction of smallmouth bass, which is presently one of <br />two dominant sport fish in the reservoir. <br />Threadfin shad (Dorosoma oetenense) were introduced by UDWR into Wahweap Bay, <br />Lake Powell in June 1968, to provide additional forage base. Approximately 90 <br />percent of the original plant were young-of-the-year (yoy). Early sampling <br />revealed that shad had spawned their first summer and spread to Halls <br />Crossing, 73 miles uptake, by the summer of 1969. Shad were found reservoir- <br />wide by the summer of 1970. Threadfin shad quickly became the predominant <br />food item of all mayor game species in Lake Powell. <br />Striped bass (Morons saxatilis), an anadromous marine species originating on <br />the Atlantic coast, were stocked into Lake Powell between 1974-1919. <br />Information available at that time did not predict that striped bass could <br />reproduce in sufficient numbers to maintain a recreational fishery in the <br />freshwater reservoir without supplemental stocking. However, unprecedented <br />natural reproduction and survival of striped bass allowed them to become the <br />mast numerous sports fish and dominate the fish community of Lake Powell. <br />Striped bass introduced into lake Powell utilized the dense pelagic shad <br />population. Threadfin shad numbers subsequently declined due to predation <br />from striped bass. Low shad density caused striped bass condition to decline <br />and resulted in starvation and reduction of striped bass. Less striped bass <br />predation allowed shad to recover. This classic predator-prey cycle <br />fluctuated wildly in the 1980's and still occurs within narrower parameters. <br />Maintaining a balance of these two species is imperative for the overall <br />quality of the sport fishery. <br />Other species now common in the reservoir (walleye, bluegill, green sunfish, <br />carp and channel catfish) developed from stock present in the river system <br />when the dam was closed. Nonnative fish living in the river that could not <br />cope with the altered reservoir environment include fathead minnow, <br />mosquitofish, and plains killifish. These fish are still present in the <br />recreation area but are found mainly in the river reaches, inflows, and small <br />12 <br />